Currently, in the field of information processing and telecommunications, an improvement of communication traffics communicating high-capacity data using the optical technology is rapidly progressing, and there have been developed optical fiber networks such as a backbone, Metro, and Access for long distances of a few kilometers and more. It would be effective in the future to use optical wirings for processing a large volume of data without delay for signal transmission between communication devices (over the distance from several meters to several hundreds meters), and even for communications over an extremely short distance (in the range from several centimeters to several tens meters) within a device.
When an optical wiring is used for communication within a device such as, for instance, a router/switch device, high-frequency signals transmitted from external networks such as Ethernet via an optical-fiber is input to a Line card. Multiple sheets of line cards are used for each backplane. The signals input to each of the line cards are concentrated on switch cards via the backplane, processed by an LSI (large scale integration) included in the switch card, and are output to each line card via the backplane. Conventionally, the signals of 300 Gigabits per second have been concentrated on the switch card via the backplane. To transmit the signals through the conventional electrical wiring, segmentation of the signals into about one to three Gigabits per second for each wiring is required, because of a propagation loss, and thus, 100 and more wirings are required.
Furthermore, the high-frequency lines need a waveform shaping circuit and require countermeasures against such problems as reflection and crosstalk between the wirings. Because there is the tendency for growth in system capacity, it is anticipated that the problems including increase of the number of wirings and crosstalk between wirings will become more serious in a device using the conventional electric wirings for processing a large volume of information at a processing speed of T bit/s or more. As a solution for the problems, it is effective to replace the conventional signal transmission lines between the line cards, the backplane, and the switch card with the optical technology. With the optical technology, the high-frequency signals of 10 Gigabits and more can be propagated with a reduced loss, and a less number of wirings are required with the necessity for countermeasures against the high-frequency signals eliminated, and therefore the optical technology is very promising.
To realize the high-capacity optical interconnection circuit as described above, it is necessary to develop an easy-to-manufacture integrated circuit board which allows for high density integration of optical wirings and optical coupling with a low loss. For the high density integration of wirings, it is effective to laminate a number of optical wiring layers such as optical waveguide arrays two-dimensionally arranged in the thickness direction of the substrate and optically connect the optical wiring layers to a surface light-emitting (or receiving) type photonic device array, because integration of wiring at a higher density is possible with a smaller mounting area in this configuration.
An example of the implementation as described above in which multiple optical wiring layers and photonic device arrays can be optically connected to each other with a low loss is disclosed in Patent Document 1. As illustrated in FIG. 1 of Patent Document 1, the example includes an arrayed optical waveguide unit for optically coupling each of the multiple wiring layers to the photonic device array, so that the light from the optical wiring layer is transferred via a core of the optical waveguide for optical coupling and thus a reduction in efficiency of the optical coupling caused by a beam spread is avoided.
Furthermore, Patent Document 2 describes another example which suppresses a reduction in efficiency of the optical coupling caused by a beam spread. As illustrated in FIG. 1 of Patent Document 2, in the example, a light beam going out from a light-emitting device is collimated by mounting a micro-lens in each of the multi-layer optical waveguide and the light-emitting elemental device, and furthermore the light beam is collected by the micro-lens and is introduced into the core of the optical waveguide. In this example, a radiation loss caused by the beam spread from the optical device is suppressed to avoid reduction in the efficiency in the optical coupling.
Patent Document 1: JP-A-2003-114365
Patent Document 2: JP-A-2001-185752